Structural studies of sapote ( Sapota achras) gum

Abstract

Sapote gum contains residues of L-arabinose (pyranose and furanose), D-xylose, D-glucuronic acid, and 4- O-methyl- D-glucuronic acid in the ratio 1.0:2.8:0.48:0.52. The two uronic acids were conveniently determined by reducing the carboxyl functions with lithium borohydride and measuring the ratio of D-glucose to 4- O-methyl- D-glucose. Periodate oxidation of the carboxyl-reduced gum gave inter alia 2- O-methyl- D-erythritol and 4- O-methyl- D-glucose in amounts suggesting that 37% of the 4- O-methyl- D-glucuronic acid residues are unsubstituted in the polysaccharide. Acetolysis of the carboxyl-reduced gum gave O-α- D-glucopyranosyl-(1→2)-(4- O-β- D-xylopyranosyl) 0,1,2,- D-xylose, a hitherto undescribed series of oligosaccharides, together with 2- O-(4- O-methyl-α- D-glucopyranosyl)- D-xylose. Methylation confirmed that sapote gum has a highly branched structure, and commercial xylanases did not depolymerize the gum. An α- L-arabinofuranosidase liberated a substantial part of the arabinose residues. Sapote gum is a member of the uncommon class of plant gums having a D-xylose backbone and structurally resembles brea gum.